1. Field of the Invention
The present invention relates to a control device and method for an internal combustion engine comprising an EGR (Exhaust Gas Recirculation) device and an intake throttle valve, and in particular to a control device and method for the internal combustion engine for controlling an intake air volume and an EGR volume.
2. Description of the Related Art
In a diesel engine, an EGR device is generally known as a device for reducing amount of NOx (nitrogen oxide) emission. This EGR device comprises an EGR passage connecting an intake passage and an exhaust passage with each other, and an EGR valve which is provided in the EGR passage.
In the EGR device, the EGR valve is opened to allow some of the exhaust gas in the exhaust passage to flow into the intake passage through the EGR passage. The some of the exhaust gas is mixed with the intake air in the intake passage and then the mixture is led into a combustion chamber. Thereby, oxygen concentration in the intake air is decreased, combustion temperature is lowered, and amount of NOx emission is reduced.
On the other hand, it is known to provide an intake throttle valve in the intake passage of diesel engine in order to ensure a prompt engine stop and a reduction of intake noise in a specific operational state.
In recent years, a diesel engine which comprises both the EGR device and the intake throttle valve is provided. In this case, the intake throttle valve is provided in the intake passage at an upstream side of a connection part with the EGR passage.
This type of the internal combustion engine provided with the EGR device and the intake throttle valve is disclosed in Japanese Laid-Open Patent Application No. 9-310633, etc.
In the conventional diesel engine which is provided with the EGR device and the intake throttle valve, it is a normal way that the intake throttle valve is controlled to adjust intake air volume, and the EGR valve is controlled to adjust EGR volume (or EGR rate).
However, in the conventional control device in which the intake air volume and the EGR volume are separately controlled, there is a possibility that a preferable control cannot be performed because the opening degrees of the intake throttle valve and the EGR valve affect both intake air volume and EGR volume.
This is described in more detail. The opening degree of the intake throttle valve affects the EGR volume as well because it changes the negative pressure state at the downstream side of the intake throttle valve in the intake passage. Moreover, since the opening degree of the EGR valve defines the EGR gas volume which is recirculated to the intake passage, it also affects the fresh intake air volume in the intake passage at the downstream side of the connection part with the EGR passage.
That is, the opening degrees of the intake throttle valve and the EGR valve, and the intake air volume and the EGR volume are closely related to each other. Accordingly, there is a limit in the conventional control device which controls them separately.